An electromagnetic flow meter (EMFM) measures the flow rate in a conductive fluid by inducing a magnetic field in the flow duct through which the fluid passes. The electrical potential induced in the fluid is a function of, and directly proportional to, the fluid velocity within the duct. While prior art EMFMs are generally capable of measuring steady-state or quasi-steady state flow rates of the fluid in the flow duct, they fail to accurately measure continuously varying velocities or flow rates. Continuously varying velocities may occur in certain flow systems, such as those used in dynamic transfer function (DTF) testing of components utilized in liquid fueled rocket propulsion systems. DTF testing is performed to characterize the dynamic response of such components in order to provide the building blocks of a fluid dynamic model of such a liquid fueled propulsion system.
Prior art EMFMs utilize pulsed direct current (DC) excitation or alternating current (AC) excitation methods to capture flows rates. The excitation is typically measured to provide normalization for the small variations in excitation voltage. The DC excitation method is limited by the pulse frequency, which limits the method to capturing quasi-steady state flow rates thereby rendering the method unsuitable for measuring continuously varying flow rates over wide bandwidths. The DC excitation method is also inherently limited by the natural charge of the conductive fluid and changes in the electrodes over time, commonly termed galvanic drift. In some situations, the electric potential induced by galvanic drift is significantly greater than the electric potential associated with the flow velocity of the fluid.
Attempts to overcome the deficiencies of the DC excitation method have resulted in the development of the AC excitation method. In the AC excitation method, the magnetic coils of the EMFM are excited with an alternating current typically powered by line (240V or 120V, 60 Hz or 50 Hz) sinusoidal voltage. The excitation is typically measured to provide normalization for the small variations in line voltage. The output signal is then measured on a mean level (root mean square or other) to provide a proportional output value to fluid flow rate. Prior efforts to capture fluctuating flow rates attempted to extend the response bandwidth of EMFM by increasing the AC excitation frequency to the point of satisfying the Nyquist criteria for the measurement of flow fluctuations. In order to adequately resolve the velocity variation of higher-frequency flow fluctuations, it was expected to require at least 10 to 25 samples throughout each flow velocity variation. However, this could require high AC frequencies that could result in high eddy current losses in the coil windings and cores. Moreover, in order to achieve adequate output signal amplitude at high measurement rates, high excitation voltages are required. These high losses on high excitation levels result in external heating of the EMFM, associated damage to the measurement device due to the heating, and associated heating of the contained fluid.